The fact that one person in the protected class has lost out to another person in the protected class is thus irrelevant, so long as he has lost out because of his age. Or to put the point more concretely, there can be no greater inference of age discrimination (as opposed to “40 or over” discrimination) when a 40 year-old is replaced by a 39 year-old than when a 56 year-old is replaced by a 40 year-old….

In the age-discrimination context, such an inference cannot be drawn from the replacement of one worker with another worker insignificantly younger. Because the ADEA prohibits discrimination on the basis of age and not class membership, the fact that a replacement is substantially younger than the plaintiff is a far more reliable indicator of age discrimination than is the fact that the plaintiff was replaced by someone outside the protected class.